Dynamic mechanical analysis of polymeric systems of pharmaceutical and biomedical significance
Introduction
Modern polymeric biomaterials have found widespread applications in medical and veterinary sciences, particularly as components of drug delivery systems and medical devices. It is accepted that the rheological (mechanical) properties of biomaterials directly affect their clinical performance (Jones et al., 1997a, Jones et al., 1997b). Therefore, in the development of both pharmaceutical and biomedical systems, it is important to accurately and usefully characterise their rheological properties to ensure optimisation of their design, and hence performance.
There are several techniques that may be employed to rheologically/mechanically characterise polymeric systems (Craig and Johnson 1995). These include:
- 1.
Thermomechanical analysis, in which a non-oscillatory stress is applied to the sample and the resultant deformation (strain) measured as a function of temperature. One example of this technique is creep analysis.
- 2.
Thermodilatometry, in which the sample is exposed to a range of temperatures and changes in the dimensions of the sample recorded.
- 3.
Dynamic mechanical analysis, a non-sample destructive technique in which an oscillatory stress is applied to the sample and the resultant strain determined as a function of both frequency and temperature. Examples of this technique include thermal-ramped oscillatory rheometry and conventional dynamic thermal mechanical analysis. Dynamic mechanical methods enable accurate and rapid quantification of the viscoelastic properties of pharmaceutical and biomedical systems
At this point, it may be useful to provide definitions of the commonly employed terms in dynamic mechanical analysis (Ferry, 1980, Barnes et al., 1996). These are:
Stress, the force per unit area (Pa) required to deform the sample.
Strain, the amount by which the sample is deformed. In dynamic mechanical analysis the strain is termed amplitude (distance), due to the vertical nature of the deformation, whereas, in oscillatory rheometry, the strain is horizontal (torsional) and is measured in radians.
Damping, the ability of a material to dissipate applied mechanical energy into heat (dimensionless quantity).
Modulus, the resistance of a materials to deformation (Pa).
Dynamic mechanical techniques have been widely employed in the polymer, and related industries, however, the applications of such techniques for the characterisation of pharmaceutical and certain biomedical systems have not received similar attention. In light of this, the aims of this review are, firstly, to describe the theoretical and practical basis of dynamic mechanical techniques and, secondly, to provide an overview of reported, and future, applications of the techniques for the characterisation of pharmaceutical and biomedical systems.
Section snippets
Theory of dynamic mechanical analysis
Dynamic mechanical methods characterise the viscoelastic properties of materials as functions of both frequency of applied oscillatory stress (or strain) and temperature. The term viscoelastic has been defined as the ‘simultaneous existence of viscous (liquid) and elastic (solid) properties in all materials’ (Barnes et al., 1996). The rheological behaviour of ideal solids is mathematically described by Hooke’s Law, which states that the strain of a sample is directly proportional to the applied
Pharmaceutical and biomedical applications of dynamic mechanical analysis
Dynamic mechanical analysis is a versatile technique that may be used to simultaneously characterise both the rheological and thermal properties of a wide range of sample types. In particular, the development of modern sample clamping systems (geometries) allows for the rapid thermorheological characterisation of many pharmaceutical and biomedical systems, ranging from viscoelastic liquids to rigid viscoelastic polymeric films. Typically, the following information concerning polymeric systems
Dynamic thermal analysis of polymeric gel systems
A gel may be defined as a cross-linked polymer network that is swollen in a liquid solvent (Ross-Murphy, 1995). Pharmaceutical and biomedical applications of gel systems include, topical drug delivery systems (Jones et al., 1997a), electrically-conductive interfaces (Jones et al., 1997c), implantable drug delivery systems (Rao et al., 1994) and as artificial body fluids, e.g. aqueous and vitreous humour. Dynamic mechanical methods have been suucessfully employed to characterise the
Dynamic thermal analysis of solid polymeric systems of pharmaceutical and biomedical significance
Solid polymeric systems are extensively employed in the design of pharmaceutical and biomedical systems. For example, polymeric films are used as components of packaging systems, as coatings of solid dosage forms and as integral layers of transdermal drug delivery systems and wound dresings, whereas, many medical devices (e.g. dental prostheses, catheters, ureteral stents) are exclusively solid (viscoelastic) polymeric systems. In many of these examples, the mechanical properties of the solid
Conclusions
This review has concisely described both the theory of dynamic mechanical analysis and, additionally, the applications of this technique for the characterisation of the thermorheological properties of pharmaceutical and biomedical systems. In particular, information may be derived concerning, polymer viscoelastic properties as functions of temperature, glass transition temperature(s), sol–gel transitions, rate and extent of polymer curing, polymer morphology and polymer-polymer compatibility.
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